Calcium carbonate and titania are each individually leading examples of white opacifiers in the field of pigmentary materials. The effects produced by these materials are due to the combined effects of a number of their physical properties, especially, particle size, morphology, and refractive index. Because these materials individually produce favorable results as opacifiers, it was decided to investigate whether the two compounds could be chemically combined in a way that produces synergistically greater benefits as an opacifier than either of the individual materials.
As a result of this research, it has been discovered that the two compounds can be chemically combined to produce high purity synthesized kassite, a calcium titanate hydrate. Kassite is known to occur naturally in trace quantities. The material was discovered in 1959 in the Kola Peninsula of the Soviet Union, but was not identified until 1965. In the U.S., small amounts of the mineral have been found at a quarry in Arkansas.
Mitsuhashi and Watanabe mentioned the formation of kassite at a temperature below 310.degree. C. using hydrothermal techniques to prepare synthetic brookite from coprecipitates of TiCl.sub.4 and CaCl.sub.2 solutions (9 Mineralogical Journal 4, pp. 236-240, Oct. 1978). They did not, however, discuss the crystallographic structure of the kassite, disclose its existence in alternative forms having different crystal morphologies, or suggest its suitability as either a pigment or a reinforcing material.
Samples of naturally occurring kassite from the Arkansas site in the U.S. were reported by Evans and Dwornik to have the hexagonal shaped platy morphology (71 American Mineralogist 1045-1048, 1986). They did not report the alternative lath form, which presumably is not a naturally occurring form.
Rudashevskii et al. reported the synthesis of kassite, but with the presence of Na, Fe, Al and Si atoms in the crystal lattice structure and not as a high purity material as is prepared in accordance with the present invention. (Rudashevskii, L.S.; Firfarova, I.B.; and Tsekhovolskaya, D.I., 106 Zap. Vses. Mineral O. Va. (1) pp. 114-20, 1977). These authors, however, also did not disclose the crystallographic structure or physical properties of the kassite and did not disclose or suggest its utility, in the alternative pure hexagonal and lath forms, as a pigment and a reinforcing material, respectively.
It is well known to use particles having fibrous or rod-like crystal structures as reinforcing materials in plastics and ceramics. Heretofore, however, it has not been known to use a calcium titanate hydrate in this capacity. The present discovery of a method of preparing a form of kassite having the lath or rodlike morphology which renders it useful as a reinforcing material represents the discovery of a completely new form of material and a new use for this material.
It was noted initially that the product kassite has a hexagonal plate morphology. Because there are no particulates represented by this morphology in existing calcium carbonate and titania pigment product lines, it was recognized that kassite produced in this way has excellent potential as a new form of pigment.
There is a great need for laminar pigments in the field of coating materials for such applications as paint finishes and paper coatings. Laminar pigments are utilized in these applications because they impart a high degree of surface smoothness. Laminar pigments are unique in that while thy have a novel particle morphology, they have a particle size distribution and exhibit a range of refractive index characteristic of white opacifying pigments.
Experiments to determine the optimum reaction conditions for reacting calcium carbonate and titania to produce the hexagonal plate form of kassite led to the production of a second form of the product having an alternative morphology. This new material, which we refer to as the "lath" form of kassite, was observed to have a fibrous or rodlike crystal structure.